This invention relates to a method for enriching the exhaust gases of a combustion engine with unburnt hydrocarbon, said combustion engine comprising a plurality of cylinders, at least one exhaust manifold for receiving exhaust gases from at least two of said plurality of cylinders, and an EGR circuit for supplying exhaust gases from said exhaust manifold to an air inlet of at least one of said cylinders, wherein said exhaust manifold comprises a first outlet for supplying exhaust gases to an exhaust aftertreatment system, and a second outlet for supplying exhaust gases to said EGR circuit.
The invention also relates to a computer program comprising program code means, a computer program product comprising program code means stored on a computer readable medium, as well as a computer system for implementing said method for enriching the exhaust gases of a combustion engine with unburnt hydrocarbon.
The inventive method is particularly applicable for diesel engines where regeneration of the exhaust aftertreatment system periodically may be needed.
The diesel engine is known for operational reliability and low fuel consumption but does not produce as low emissions as, for example, a petrol engine provided with a three-way catalyst. One way of improving the emissions from a diesel engine is to fit a particle filter which filters soot and particles from the exhaust gases and/or the NOx post-treatment system. These filters are usually very effective and gather both large and small particles. To prevent the filter from becoming full of soot and causing a major pressure drop for the exhaust gases leaving the engine, the soot has to be burnt. One method is for this soot to be burnt by the nitrogen oxides contained in diesel exhaust gases. In that case, the portion of the nitrogen oxides that takes the form of NO2 can oxidise the soot within the temperature interval of about 250 to 400 Degrees centigrade but this process takes a relatively long time and needs to be more or less constantly active even if there is an oxidation catalyst before the filter or the filter itself is covered with a catalytic layer. Another method for oxidising soot accumulated in the particle filter is to heat the filter to about 600-650 degrees centigrade so that the surplus oxygen O2 from diesel combustion can oxidise the soot directly, which is a rapid process. Soot accumulated over many hours of operating time can be oxidised away in a time of the order of 5 to 10 minutes. The exhaust temperature of a diesel engine normally never reaches 600-650 Degrees centigrade particularly after a turbo unit whereby the turbine extracts power from the exhaust flow and causes a temperature drop. It is not unusual that the exhaust temperature after the turbine of the turbo unit is lower than 250 Degrees centigrade, which is a temperature below which an oxidation catalyst does not function properly.
Temporarily increased temperature of the exhaust for regeneration of a NOx trap, such as a particle filter, may for example be realised by supplying unburnt hydrocarbon, e.g. in the form of fuel into the exhaust gas upstream of the NOx trap or NOx catalyst. One common solution is to provide the exhaust system with a separate fuel injector for injecting unburnt hydrocarbon directly into the exhaust system upstream of the particulate filter. This solution however requires an additional fuel injector located within the exhaust system, as well as fuel supply lines to said additional fuel injector, thereby increasing the cost, maintenance requirement, and risk of malfunction of the exhaust aftertreatment system.
According to an alternative solution, unburnt hydrocarbon is supplied by in-cylinder post injection of fuel into one or more cylinders. This solution thus does not require an additional fuel injector for providing unburnt hydrocarbon to the exhaust gas. Some of the most modern diesel engines are often equipped with exhaust gas recirculation (EGR) to reduce emissions of nitrogen oxides. Combining in-cylinder post injection, EGR system, and particle filters and/or NOx post-treatment by so-called NOx trap or NOx catalyst entails complications, such as potential contamination of the EGR circuit by unburnt hydrocarbon, as well as enriching the intake air with the unburnt hydrocarbon, possibly resulting in altered combustion conditions. There are therefore gains to be made by channeling exhaust gases with unburnt hydrocarbon to the exhaust system for regeneration purposes, but preventing unburnt hydrocarbon from being recirculated in the EGR circuit.
These adverse effects due to fuel in the EGR circuit can be prevented by selectively closing the EGR circuit when late post-injection takes place, as disclosed in US20060196178 A1, or by applying in-cylinder post injection only in carefully selected cylinders in combination with a specifically designed exhaust manifold, as disclosed in US2008110161, or by providing an exhaust system that is divided into two portions in combination with in-cylinder post injection in certain designated cylinders, as disclosed in U.S. Pat. No. 5,987,884 and U.S. Pat. No. 6,141,959.
The method of controlling exhaust gas flow by means of controllable valve means or internal partition walls dividing the exhaust manifold and/or system into at least two portions results in a more expensive exhaust system, and requires more space for installation. A specifically designed exhaust manifold however requires significant design effort and limits the possible design possibilities of the exhaust manifold. There is thus a need for an improved method for enriching the exhaust gases of a combustion engine with unburnt hydrocarbon that removes the above mentioned disadvantages.
It is desirable to provide a method for enriching the exhaust gases of a combustion engine with unburnt hydrocarbon where the previously mentioned problem is at least partly avoided.
The invention concerns, according to an aspect thereof, a method for enriching the exhaust gases of a combustion engine with unburnt hydrocarbon, said combustion engine comprising a plurality of cylinders, at least one exhaust manifold for receiving exhaust gases from at least two of said plurality of cylinders, and an EGR circuit for supplying exhaust gases from said exhaust manifold to an air inlet of at least one of said cylinders, wherein said exhaust manifold comprises a first outlet for supplying exhaust gases to an exhaust aftertreatment system, and a second outlet for supplying exhaust gases to said EGR circuit.
The invention is characterized, according to an aspect thereof, by the step of monitoring engine operating conditions, and performing in-cylinder post injection of said unburnt hydrocarbon into one predetermined cylinder of said at least two cylinders connected to the exhaust manifold when the monitored engine operating conditions equal predetermined engine operating conditions, which predetermined engine operating conditions are set for resulting in a substantially zero flow of said post-injected hydrocarbon to the EGR circuit for the specific design of the engine in operation.
One advantageous effect of the method according to an aspect of the invention is the possibility to omit any fuel injector arranged in the exhaust system for the sole purpose of regenerating parts of the exhaust aftertreatment system by heating, and instead using one or more of the existing combustion cylinders fuel injectors for this purpose. Omission of a fuel injector within the exhaust system results in reduced costs and increased reliability of the engine and exhaust system. A further advantage is a high degree of separation of exhaust gas from cylinder in which post injection of fuel is performed and exhaust gas entering the EGR circuit, without expensive or space-consuming measures. Furthermore, the inventive method allows simultaneous in-cylinder post injection of fuel and active EGR operation, such that a continuously high level of NOx reduction by the EGR system can be provided.
Further advantages are achieved by implementing one or several of the features of the dependent claims.
A combination of said one predetermined cylinder and said predetermined engine operating conditions may be based on a mapping and/or modelling from which cylinder the exhaust gases flowing through said EGR circuit origins at different engine operating conditions. In other words, the mapping and/or modelling is used to determine from which individual cylinder the exhaust gas flowing in the EGR circuit origins and thereby to determine which cylinder the in-cylinder post injection of fuel advantageously should be conducted for a specific engine operating condition. Upon determining that the exhaust gas from one or more cylinders not at all, or only to a small degree, enters the EGR circuit, then this one or more cylinders can be registered as suitable for said fuel post injection upon registering required predetermined engine operating conditions.
It may be preferred to perform said in-cylinder post injection of said unburnt hydrocarbon into said one predetermined cylinder only when said monitored engine operating conditions equal said predetermined engine operating conditions, because then substantially normal operation of the EGR circuit can be realised.
The performance of in-cylinder post injection of said unburnt hydrocarbon into said one predetermined cylinder may be the only means for enriching the exhaust gases of said combustion engine with unburnt hydrocarbon and at the same time avoiding that the post-injected hydrocarbon reaches the EGR circuit.
The engine operating conditions may preferably comprise at least engine speed and engine load. An additional or alternative engine operating condition comprises exhaust gas flow rate in said EGR circuit, or a corresponding parameter.
The EGR circuit may be provided with a single EGR valve for each exhaust manifold, or for each set of cylinders, or for each engine. Conventional EGR systems uses a single EGR valve to control the EGR flow rate, and additional control valves for preventing unburnt hydrocarbon from entering the EGR circuit leads to increased costs and reduced reliability of the engine, as well as increased maintenance effort.
The first and second outlets may be positioned at a distance from each other in a direction parallel with an alignment direction of the cylinders connected to said at least one exhaust manifold. Separation of the first and second outlets in said direction increases the amount of engine operating conditions in which fuel post injection can be performed without too high level of unburnt hydrocarbons entering the EGR circuit.
The exhaust manifold may preferably be free from internal partition walls and/or controllable flow valves.
The present invention further provide, according to an aspect thereof, a computer program comprising program code means for performing all the steps of the inventive method described above when said program is run on a computer.
The present invention further provides, according to an aspect thereof, a computer program product comprising program code means stored on a computer readable medium for performing all the steps of the inventive method described above when said program product is nm on a computer.
The present invention further provides, according to an aspect thereof, a computer system for implementing the method for enriching the exhaust gases of a combustion engine with unburnt hydrocarbon, said combustion engine comprising a plurality of cylinders, at least one exhaust manifold for receiving exhaust gases from at least two of said plurality of cylinders, and an EGR circuit for supplying exhaust gases from said exhaust manifold to an air inlet of at least one of said cylinders, wherein said exhaust manifold comprises a first outlet for supplying exhaust gases to an exhaust aftertreatment system, and a second outlet for supplying exhaust gases to said EGR circuit, the computer system comprising a processor operable to monitor engine operating conditions and commanding in-cylinder post injection of said unburnt hydrocarbon into one predetermined cylinder of said at least two cylinders connected to the exhaust manifold when the monitored engine operating conditions equal predetermined engine operating conditions, which predetermined engine operating conditions are set for resulting in a substantially zero flow of said post-injected hydrocarbon to the EGR circuit for the specific design of the engine in operation.